Stepper motors are an important part of many products, such as sheet and insert material handlers used in mail piece inserters. The size of stepper motors required in such devices is directly related to cost. Further, performance issues are intertwined with cost issues for mail piece inserters, as well as other similar devices.
Small stepper motors, on the order of 10-15 watts, are generally more cost-effective and are usually available in many off-the-shelf configurations. Normally, the maximum torque required for a given application dictates the size of the stepper motor used in that application. That is, for a stepper motor used to drive a feed roller, the required torque is typically related to the thickness of the material fed by the feed roller. Feeding thick materials requires more torque compared to feeding thinner materials. Accordingly, even if most feed situations require less torque, normal design criteria calls for a stepper motor capable of meeting the highest torque demands.
As torque requirements increase, the drive current for a stepper motor also increases. High drive current produces a significant amount of heat which can lead to an overheating condition if the motor is energized for long periods of time. As such, the motor's duty cycle, or the amount of time the stepper motor is energized to drive a particular roller or gear, is an additional consideration to obviate overheating conditions. Consequently, when driven to produce high torque, small stepper motors can generate excessive localized heat. It then becomes difficult to address the buildup of excessive heat when such a stepper motor is located in an enclosed or confined area such as a document feed device or folding unit.
Small stepper motors generally produce a significant amount of heat at the high end of its torque range in contrast to larger stepper motors. However, it is typically cost effective to utilize such stepper motors in lightweight, compact consumer and office electronics. As such, it has becomes important to reduce the amount of heat generated by and/or increase the heat transfer from interior stepper motor assemblies while maintaining the ability to produce high levels of torque when necessary.
Several methods have been employed to address the difficulties of such heat generation when small stepper motors have a high duty cycle and operate at the high end of their torque range. These methods principally augment the heat transfer from the stepper motor assembly to the surrounding ambient environment in an effort to reduce the overall operating temperature. However, in sheet handling devices such as mail piece inserters, where package miniaturization is important, these methods become impractical. Further, it is equally impractical to reduce the stepper motor duty cycle in an effort to reduce heat because feed-throughput is a competitive advantage.
Therefore, new methods are needed to reduce the various modes of heat-related failure that occur during high torque operation. New methods are also needed to address the heat generated when the driving current approaches the thermal limits of the stepper motor. Further, a need exists for an improved stepper motor control which is useful in space-limited confined areas, such as mail inserters and similar products.